Microelectromechanical system (MEMS) devices have achieved a great attention in last decade due to their use in many high volume smart systems, such as smart phones, automobiles, and various biomedical devices and applications. Most of the MEMS devices cannot be used stand alone and need integration with Application Specific Integrated Circuits (ASIC) for driving and/or sensing the MEMS device output.
Integration of MEMS device with electronic circuitry can be divided into three categories:
1) Two dimensional (2D)-Multi-chip Solution: In this type of integration scheme both MEMS and ASIC are fabricated separately on different substrates using dedicated MEMS and IC processes, and then connected by placing MEMS and IC chips side by side during final hybridization. 2D multi-chip solutions may be known as system on board, or system in package (SiP);
2) Vertical Multi-chip solution: In this integration scheme, separately fabricated MEMS and IC devices are integrated vertically either using chip-to-chip, chip-to-wafer or wafer-to-wafer bonding techniques; and
3) Monolithic Integration: In this integration scheme, both the MEMS device and electronic circuitry are fabricated on the same substrate. Monolithic integration may be further split into four sub techniques: a) MEMS first monolithic integration, b) MEMS and IC interlaced processing, c) MEMS last processing with bulk micro-machining, and d) MEMS last processing with surface micromachining.
Each type of the MEMS to ASIC integration has pros and cons. High parasitic and poor connection density are of concern in multichip solutions, while cumulative yield and MEMS device performance could be a concern in monolithic integration, if the monolithic integration is not properly designed. In some cases, such as 2D piezoelectric micromachined ultrasonic transducer (PMUT) arrays for finger printing applications or medical imaging, only monolithic integration is capable to produce required resolution. Other schemes fail to achieve required array density due to large bond pad size requirement, as each transducer in the array need to be controlled by electronics independently.